This invention relates generally to screens and more particularly concerns screens used for removing undesirable particles from a liquid such as oil well drilling fluids.
The factors of primary importance in vibrating screens are particle separation, fluid throughput or conductance and screen life. Finer particle separation results in a higher percentage of impurities being removed from the screened fluid. Higher conductances are desirable because more fluid can be processed per square foot of screen area, thereby reducing costs. Doubling the conductance doubles the liquid throughput. Longer screen life saves time and money. Since the mid-seventies the vibrating screen industry trend has been to decrease wire diameter in order to achieve higher conductance. The result has been finer separation and higher conductance but shorter screen life. In order to increase screen life, the industry has resorted to various types of bonded screens such as plastic-backed, metal-backed or bonded-backup, but bonded screens are more expensive.
In order to maintain high conductance and screen life, I have developed double shute or warp screens which are described in my U.S. patent application Ser. Nos. 07/870,486, filed Apr. 16, 1992, which is now U.S. Pat. No. 5,256,291, and 07/893,715, filed Jun. 5, 1992, which is now U.S. Pat. No. 5,256,292. For a double warp plain weave screen having warp and shute wires of the same material and properties, the shute diameter must be at least 1.4 times the warp diameter to prevent sleaziness. Since the shute diameter controls the conductance, if the shute diameter is fine enough to give very high conductance, the warp diameter is so fine that the screen has a low tensile strength and therefore shorter life.
Thus, while the double warp screens described in my earlier patent applications provide improved conductance and life over the previous art, an undesirable compromise between separation, conductance and life is often necessary for finer screens having very high ranges of conductance.
Coarse triple warp plain weave screens have been used in the past, ranging from very coarse up to about 50-60 mesh. The aspect ratios of the rectangular openings of these known triple warp screens vary from about 4/1 to 10/1 but the ratios of their shute to warp diameters are 1.5/1 or greater. Use of these higher diameter ratio screens decreases conductance to such unacceptable levels that known triple warp screens cannot be used in many finer mesh applications.
It is, therefore, a primary object of this invention to provide a vibrating screen composed of fine wire in the order of approximately 0.0010" to 0.0070" which provides finer particle separation, higher conductance and longer screen life than presently known screens.